415
catfi sh in the USA, regarded as leading to more mortalities than group B (Soto et al.
2007 ) . Genetic differences between high and low virulent strains have been determined by suppression subtractive hybridization, and include ferrous iron transport
protein, TonB-dependent receptor, transposases and ABC transporter permease protein (Li et al. 2010 ). Virulent and non-virulent cultures display variation in colony
morphology and chondroitin AC lyase activity with the former being rhizoidal,
moderately adherent [temperature dependent, with increased adhesion up to 20 °C]
with higher chondroitin AC lyase activity whereas the latter are non-rhizoidal and
rough with less chondroitin AC lyase activity or soft colony types which are poorly
adherent (Kunttu et al. 2011 ). However, use of gene deletion mutants revealed that
chondroitin lyases were not essential virulence factors (Li et al. 2015a ).
Pacha and Ordal ( 1963 ) classifi ed cultures into four grades of virulence, from
high to low. Using a highly virulent culture, it was possible to achieve infection of
chinook salmon and sockeye salmon following a 2-min dip in a diluted broth culture. Possibly, the organism entered the host through damaged areas of skin, especially if physically abraded or hot branded (Bader et al. 2003b , 2006 ), although it
recognized that Fla. columnare attaches to the gill. Thus using a common carp gill
perfusion approach with bivalent ion-rich water, the presence of nitrite or organic
material, and high temperatures (=28 °C), it was noted that a highly virulent culture
adhered more readily than a culture of low virulence (Decostere et al. 1999a ). The
adherence receptor was considered to be composed at least partially of carbohydrate, with the adherence ability of Fla. columnare correlated with haemagglutination and the capsule (Decostere et al. 1999b ). Fujihara et al. ( 1971 ) achieved 100 %
mortality in chinook salmon following exposure for 25 min in a suspension containing 2.5 × 10
5 cells/ml. Deaths followed within 96 h and 8 h at water temperatures of
10 and 22 °C, respectively. Adhesion and infection is inversely proportional to the
salinity, specifi cally there was a decline in mortalities from freshwater (98 % mortalities) to 3‰ (0 % mortalities) for channel catfi sh (Altinok and Grizzle 2001 ).
Also, the concentration of bivalent, i.e. calcium and magnesium, cations in water
infl uences pathogenesis with removal by ion-exchange fi ltration decreasing fl avobacterial adhesion to gill tissue and thus mortalities (Straus et al. 2015 ). However,
the pathogenic mechanism is unclear, but work has pointed to an involvement of
adhesion principally insofar as an adhesion-defective mutant had reduced virulence
(Bader et al. 2005 ). A preceding stage in an infection cycle is the arrival of the
pathogen at the cells/tissues. Whereas, the event may be random, in many cases
there is a direct attraction of the pathogen to the host cells. There is evidence that
Fla. columnare is attracted chemotactically to skin mucus, with carbohydratebinding receptors important in the process (Klesius et al. 2010 ). Using a high virulent culture, there was a high level of colonization and destruction; invasion of gill
tissue was observed to spread from the fi lament tips towards the base, with outer
membrane vesicles surrounding most of the fl avobacterial cells. Higher virulent
cultures led to signifi cantly higher apoptotic cell counts in carp and rainbow trout
(Declercq et al. 2015 ). Using RNA-seq expression profi ling, a rhamnose-binding
lectin was found that was dramatically upregulated in gills of channel catfi sh during
infection. In a susceptible channel catfi sh family, there was a dramatic upregulation
Flavobacterium columnare (=Flexibacter/Cytophaga columnaris)
catfi sh in the USA, regarded as leading to more mortalities than group B (Soto et al.
2007 ) . Genetic differences between high and low virulent strains have been determined by suppression subtractive hybridization, and include ferrous iron transport
protein, TonB-dependent receptor, transposases and ABC transporter permease protein (Li et al. 2010 ). Virulent and non-virulent cultures display variation in colony
morphology and chondroitin AC lyase activity with the former being rhizoidal,
moderately adherent [temperature dependent, with increased adhesion up to 20 °C]
with higher chondroitin AC lyase activity whereas the latter are non-rhizoidal and
rough with less chondroitin AC lyase activity or soft colony types which are poorly
adherent (Kunttu et al. 2011 ). However, use of gene deletion mutants revealed that
chondroitin lyases were not essential virulence factors (Li et al. 2015a ).
Pacha and Ordal ( 1963 ) classifi ed cultures into four grades of virulence, from
high to low. Using a highly virulent culture, it was possible to achieve infection of
chinook salmon and sockeye salmon following a 2-min dip in a diluted broth culture. Possibly, the organism entered the host through damaged areas of skin, especially if physically abraded or hot branded (Bader et al. 2003b , 2006 ), although it
recognized that Fla. columnare attaches to the gill. Thus using a common carp gill
perfusion approach with bivalent ion-rich water, the presence of nitrite or organic
material, and high temperatures (=28 °C), it was noted that a highly virulent culture
adhered more readily than a culture of low virulence (Decostere et al. 1999a ). The
adherence receptor was considered to be composed at least partially of carbohydrate, with the adherence ability of Fla. columnare correlated with haemagglutination and the capsule (Decostere et al. 1999b ). Fujihara et al. ( 1971 ) achieved 100 %
mortality in chinook salmon following exposure for 25 min in a suspension containing 2.5 × 10
5 cells/ml. Deaths followed within 96 h and 8 h at water temperatures of
10 and 22 °C, respectively. Adhesion and infection is inversely proportional to the
salinity, specifi cally there was a decline in mortalities from freshwater (98 % mortalities) to 3‰ (0 % mortalities) for channel catfi sh (Altinok and Grizzle 2001 ).
Also, the concentration of bivalent, i.e. calcium and magnesium, cations in water
infl uences pathogenesis with removal by ion-exchange fi ltration decreasing fl avobacterial adhesion to gill tissue and thus mortalities (Straus et al. 2015 ). However,
the pathogenic mechanism is unclear, but work has pointed to an involvement of
adhesion principally insofar as an adhesion-defective mutant had reduced virulence
(Bader et al. 2005 ). A preceding stage in an infection cycle is the arrival of the
pathogen at the cells/tissues. Whereas, the event may be random, in many cases
there is a direct attraction of the pathogen to the host cells. There is evidence that
Fla. columnare is attracted chemotactically to skin mucus, with carbohydratebinding receptors important in the process (Klesius et al. 2010 ). Using a high virulent culture, there was a high level of colonization and destruction; invasion of gill
tissue was observed to spread from the fi lament tips towards the base, with outer
membrane vesicles surrounding most of the fl avobacterial cells. Higher virulent
cultures led to signifi cantly higher apoptotic cell counts in carp and rainbow trout
(Declercq et al. 2015 ). Using RNA-seq expression profi ling, a rhamnose-binding
lectin was found that was dramatically upregulated in gills of channel catfi sh during
infection. In a susceptible channel catfi sh family, there was a dramatic upregulation
Flavobacterium columnare (=Flexibacter/Cytophaga columnaris)